A problem typically encountered with the handling of cargo by a hoist system is the sway of the load which occurs due to the suspension of the load by cables, and the necessity to transport the load causing horizontal movement of the suspended load. Horizontal movement usually causes the suspended load to exhibit pendulous motion from horizontal acceleration and deceleration forces inherent in the movement of pivoting cranes, mobile gantry cranes or similar hoist systems. In addition, rotational motion of the suspended load can occur from centrifugal forces when a mobile crane is being steered within a storage area.
In many instances, accurate positioning of the suspended load for deposit and release requires undesirable waiting time to allow the swaying motion of the load to subside. The time intervals required for allowing the swaying motion of the load to cease unduly increases the time interval necessary for handling a particular load, thereby causing large amounts of idle time during the load handling function. In order to increase the efficiency of load handling, it is necessary to eliminate or reduce the time spent waiting for the swaying motion of a load to abate or cease.
If time is not spent waiting for the swaying motion of the suspended load to cease or abate, increased load stresses caused by the swaying motion can be imparted to the hoist structure upon movement of the swaying load up or down by the hoist system. Such stress is undesirable and can be potentially damaging to the hoist structure as well as the suspended load. To eliminate such undesirable stress and potential damage to the load, it is of primary importance to reduce and abate the pendulous motion of the load.
As noted, it is necessary to steady the suspended load of a hoist system in order to avoid longitudinal, transverse and swinging motions which interfere with the accurate placement of such suspended loads onto carriers and the like. For example, in the loading or unloading of cargo containers onto or off truck trailers and subsequently onto and off piggy-back railroad cars, it is necessary to position the container with respect to the trailer or railroad car to properly release and deposit the load. In order to efficiently perform such operations and to avoid undesirable delay in the loading operation, swaying or swinging of the container must be reduced.
Numerous anti-sway devices have been purposed for abating and eliminating pendulous motion of a suspended load. Typically, these anti-sway devices incorporate mechanical control mechanisms including a reel for taking up slack of a guide rope attached to the load supporting portion of the hoist or to the load itself. These prior devices also include mechanical braking mechanisms to create a dampening effect allowing the guide rope to remain taut during lowering of the load, thereby arresting pendulous motion of the suspended load. However, problems occur with use of such mechanical braking mechanisms due to the large amount of heat that is generated in creating the dampening necessary to abate the pendulous motion of the suspended load. The heat developed through mechanical braking causes undesired rapid deterioration of the mechanical braking device creating premature replacement problems and increased costs.
Another disadvantage of the previously known mechanical sway control mechanisms is the necessity of using an unduly complicated rigging system of rope or sway control cable to provide the desired sway abating function. Such complicated riggings are undesirable due to the increased costs, operating expenses and repair complications associated therewith.
Thus, it would be beneficial to provide a sway control arrangement for the hoist system that does not require a mechanical braking device. Further, it would be desirable to eliminate the requirement of a complicated rigging systems for providing the desired sway abating function.